|Artificial arms can now sense touch.|
Our ability to perceive tactile sensations is an essential part of our interaction with the world. So for decades, researchers have been working on recreating a sense of touch for people who have lost their limbs, knowing that it will help with the control and sense of embodiment of a prosthetic limb.
Teams have attempted to recreate a sense of pressure by delivering vibrations to a person's residual limb that equate to the pressure on the prosthesis. But this proved more distracting than helpful. Electrodes attached to the inside of residual nerves have also been tried, but the tingling sensations that they produce last only a few weeks before diminishing.
Now, researchers have found a way of transmitting long-term, realistic tactile sensations, such as the feel of a cotton wool ball or the trickle of water, to two people who lost hands in industrial accidents. The results were published in Science Translational Medicine. Two years ago, the team implanted a cuff of electrodes around the three main nerves in the residual part of their arms. These nerves would usually transmit sensory information from the hand to the brain. Each cuff contained up to eight electrodes, capable of individually stimulating different parts of the nerves. The electrodes were attached to wires hooked up to a machine providing stimulation via a stream of electrical pulses. This was connected to the prostheses the men were already using.
As the team switched on each electrode in turn, the men felt the sensation of touch as if it were coming from the tip of their prosthetic thumb, then the tip of the index finger and so on. The stimulation was described as tingling, tickling or pricking.
To go beyond this, the team investigated what happened when they used different frequencies and intensities of the electric pulses to produce the simulations. Previous research had given them some idea of the electrode patterns needed to recreate more complex sensations. After a healthy dose of trial and error, the team was able to deduce the pattern of electrodes required to recreate many realistic tactile sensations, such as the feeling of pressing a finger on the tip of a ball-point pen, the feeling of someone stroking a finger, the sense that a cotton wool ball being lightly rubbed on the skin and even a sense of water trickling over the back of the hand.
In order for the prosthesis to be useful, it needs to be able to deduce what sensations are appropriate for the wearer to feel in different scenarios. Sensors on the prosthetic limb go some way to achieving this, by measuring various aspects of touch and pressure which is sent to the electrodes, enabling them to fire in the correct stimulation pattern.
To test whether the natural sensation of touch improved the ability to manipulate delicate objects, the men were given a bunch of cherries. Pulling the stem from a cherry requires delicate grasp control. If your grasp is too light the cherry slips out of your hand, too heavy and you crush the cherry, see the video below.
When blindfolded, the subjects were able to use their prosthetic limb to pluck about 43 per cent of cherries without tactile feedback. Once the tactile feedback was switched on, their success rate rose to 93 percent. Sighted, their performance increased from 77 to 100 per cent. There was an unexpected health benefit, too. Before trialling the prostheses, both people had phantom limb pain, one described it as "a nail being driven through my thumb", the other said it often felt like his fist was being squeezed in a vice. Over the two years of testing, the pain diminished and eventually disappeared.
For now, both men have had to leave their sense of touch in the lab. But another amputee in Sweden has, for more than a year, been wearing a prosthesis that can sense basic touch. In early 2013, he became the first person in the world to receive a prosthesis that connected to his bone, nerves and muscles. This not only made it more comfortable than regular prostheses, but he could control movement and perceive some basic tingling sensations from things he touched. It was announced this week that a year and a half later, he is still using it successfully. The results were published in Science Translational Medicine. This is the first time a prosthesis that can perceive touch as well as movement has been worn for more than 30 days outside of a lab. There's no reason to think it will need replacing any time soon. The researchers in Sweden now hopes to use Tyler's research to help make the sensations feel more natural.